Calculate the kinetic energy of a car of mass $750\, kg$ moving with a velocity of $54\, km h^{-1}$. Find the new kinetic energy of the car if a passenger of mass $50\, kg$ sits in the car.

(N/A) Given: Mass of car $m_{c} = 750\, kg$,Velocity $v = 54\, km h^{-1} = 54 \times \frac{5}{18} = 15\, m s^{-1}$.
Initial Kinetic Energy $(KE_{i})$ = $\frac{1}{2} m_{c} v^{2} = \frac{1}{2} \times 750 \times (15)^{2} = 375 \times 225 = 84375\, J$.
When a passenger of mass $50\, kg$ sits in the car,the new total mass $M = 750 + 50 = 800\, kg$.
New Kinetic Energy $(KE_{f})$ = $\frac{1}{2} M v^{2} = \frac{1}{2} \times 800 \times (15)^{2} = 400 \times 225 = 90000\, J$.